Metabolic rate of carrying added mass: a function of walking speed, carried mass and mass location

Metabolic Costs of Walking with Weighted Vests

INTRODUCTION

The US Army Load Carriage Decision Aid (LCDA) metabolic model is used by militaries across the globe and is intended to predict physiological responses, specifically metabolic costs, in a wide range of dismounted warfighter operations. However, the LCDA has yet to be adapted for vest-borne load carriage, which is commonplace in tactical populations, and differs in energetic costs to backpacking and other forms of load carriage.

PURPOSE

The purpose of this study is to develop and validate a metabolic model term that accurately estimates the effect of weighted vest loads on standing and walking metabolic rate for military mission-planning and general applications.

METHODS

Twenty healthy, physically active military-age adults (4 women, 16 men; age, 26 ± 8 yr old; height, 1.74 ± 0.09 m; body mass, 81 ± 16 kg) walked for 6 to 21 min with four levels of weighted vest loading (0 to 66% body mass) at up to 11 treadmill speeds (0.45 to 1.97 m·s -1 ). Using indirect calorimetry measurements, we derived a new model term for estimating metabolic rate when carrying vest-borne loads. Model estimates were evaluated internally by k -fold cross-validation and externally against 12 reference datasets (264 total participants). We tested if the 90% confidence interval of the mean paired difference was within equivalence limits equal to 10% of the measured walking metabolic rate. Estimation accuracy, precision, and level of agreement were also evaluated by the bias, standard deviation of paired differences, and concordance correlation coefficient (CCC), respectively.

RESULTS

Metabolic rate estimates using the new weighted vest term were statistically equivalent ( P < 0.01) to measured values in the current study (bias, -0.01 ± 0.54 W·kg -1 ; CCC, 0.973) as well as from the 12 reference datasets (bias, -0.16 ± 0.59 W·kg -1 ; CCC, 0.963).

CONCLUSIONS

The updated LCDA metabolic model calculates accurate predictions of metabolic rate when carrying heavy backpack and vest-borne loads. Tactical populations and recreational athletes that train with weighted vests can confidently use the simplified LCDA metabolic calculator provided as Supplemental Digital Content to estimate metabolic rates for work/rest guidance, training periodization, and nutritional interventions.

Towards meaningful community ambulation in individuals post stroke through use of a smart hip exoskeleton: A preliminary investigation

Stroke is the leading cause of long-term disability in the United States, leaving survivors with profound mobility challenges that impact independent community ambulation. Evidence shows assistance at the hip during walking may be beneficial for stroke survivors. In this cross-over design study, we examine the impact of a novel hip exoskeleton on both functional and patient reported outcomes measuring speed, fall risk, gait symmetry, energy expenditure and perceived walking ability during both indoors and outdoors in single and serial counting dual task paradigms. Nine ambulatory stroke survivors with hemiplegia were included. No differences were seen between the exoskeleton and baseline conditions for any outcomes. Only the patient reported outcome in which subjects were asked to rate their ability to walk outdoors approached statistical significance (p = 0.051) with greater improvement reported for the exoskeleton condition. When asked to rate several key factors about the exoskeleton, weight and assistance emerged as primary perceived negative factors of the exoskeleton underscoring the need for improvements to the technology in this area. Despite lack of differences across groups, some individuals responded positively to the exoskeleton for several functional outcomes measured, highlighting the need for additional exploration into the use of personalized hip exoskeletons for post-stroke rehabilitation.

Metabolic power response to added mass on the lower extremities during running

BACKGROUND

Wearable exoskeletal devices can enhance locomotor performance, but their mass results in a metabolic penalty. Previous studies have quantified the metabolic cost of running with added mass on the feet, but less is known about the effects of adding mass to the thigh and shank segments.

AIM

To quantify the metabolic cost of running with additional leg mass.

METHODS

15 participants (7 F, 8 M) completed treadmill running trials (3 m/s) normally and with lead mass (300-1350 g) attached to either the thigh, shank, or foot, bilaterally. We measured metabolic power using expired gas analysis.

RESULTS

Per 1000 g of added mass per leg, gross metabolic power increased by approximately 16% (foot) and 11% (shank) for females which was slightly greater than the 11% and 8% increases for males, respectively. For thigh loading, metabolic power increased by just 4% per 1000 g in both sexes.

CONCLUSION

Adding mass more distally on the leg increases the metabolic cost of running to a greater extent. For the same absolute added mass on the foot or shank, metabolic power increases more in females.

Joint kinematic and kinetic responses to added mass on the lower extremities during running

AIM

We analyzed the biomechanical response (joint angles, moments, and powers) to running with added leg mass. These data may help guide the design of wearable locomotor assistive devices (i.e., exoskeletons), which are becoming more prevalent.

METHODS

15 participants (7 females, 8 males) completed treadmill running trials (3m•s-1) normally and with lead mass (300-1350 g) attached to the thigh, shank, or foot, bilaterally. We quantified the lower limb biomechanics combining motion capture and ground reaction force data using standard inverse dynamics analysis.

RESULTS

Only moderate kinematic changes occurred in response to the distal added limb mass. Maximum hip flexion and maximum knee flexion angles during swing phase increased by approximately 9% and 6% respectively for each 1 kg added to each foot. However, adding even small masses made dramatic changes to the joint moments and powers, mostly during the swing phase. For example, adding 1 kg to each foot increased maximum joint moments by as much as 40% (knee extension in late swing) and maximum joint power by as much as 50% (hip generation in late swing).

CONCLUSION

Leg joint kinematics were largely conserved in response to adding mass to the legs. Adding mass to the leg distally increased joint power mainly at the knee and hip joints during the swing phase, whereas adding mass proximally mainly affected the ankle joint mechanics during the stance phase. These changes have implications for shoe designs, people who run with added mass on their legs for sport/strength training and for the design of wearable devices.

Peak performance and cardiometabolic responses of modern US army soldiers during heavy, fatiguing vest-borne load carriage

INTRODUCTION

Physiological limits imposed by vest-borne loads must be defined for optimal performance monitoring of the modern dismounted warfighter.

PURPOSE

To evaluate how weighted vests affect locomotion economy and relative cardiometabolic strain during military load carriage while identifying key physiological predictors of exhaustion limits.

METHODS

Fifteen US Army soldiers (4 women, 11 men; age, 26 ± 8 years; height, 173 ± 10 cm; body mass (BM), 79 ± 16 kg) performed four incremental walking tests with different vest loads (0, 22, 44, or 66% BM). We examined the effects of vest-borne loading on peak walking speed, the physiological costs of transport, and relative work intensity. We then sought to determine which of the cardiometabolic indicators (oxygen uptake, heart rate, respiration rate) was most predictive of task failure.

RESULTS

Peak walking speed significantly decreased with successively heavier vest loads (p < 0.01). Physiological costs per kilometer walked were significantly higher with added vest loads for each measure (p < 0.05). Relative oxygen uptake and heart rate were significantly higher during the loaded trials than the 0% BM trial (p < 0.01) yet not different from one another (p > 0.07). Conversely, respiration rate was significantly higher with the heavier load in every comparison (p < 0.01). Probability modeling revealed heart rate as the best predictor of task failure (marginal R², 0.587, conditional R², 0.791).

CONCLUSION

Heavy vest-borne loads cause exceptional losses in performance capabilities and increased physiological strain during walking. Heart rate provides a useful non-invasive indicator of relative intensity and task failure during military load carriage.

Heat Stress Management in the Military: Wet-Bulb Globe Temperature Offsets for Modern Body Armor Systems

OBJECTIVE

The aim of this study was to model the effect of body armor coverage on body core temperature elevation and wet-bulb globe temperature (WBGT) offset.

BACKGROUND

Heat stress is a critical factor influencing the health and safety of military populations. Work duration limits can be imposed to mitigate the risk of exertional heat illness and are derived based on the environmental conditions (WBGT). Traditionally a 3°C offset to WBGT is recommended when wearing body armor; however, modern body armor systems provide a range of coverage options, which may influence thermal strain imposed on the wearer.

METHOD

The biophysical properties of four military clothing ensembles of increasing ballistic protection coverage were measured on a heated sweating manikin in accordance with standard international criteria. Body core temperature elevation during light, moderate, and heavy work was modeled in environmental conditions from 16°C to 34°C WBGT using the heat strain decision aid.

RESULTS

Increasing ballistic protection resulted in shorter work durations to reach a critical core temperature limit of 38.5°C. Environmental conditions, armor coverage, and work intensity had a significant influence on WBGT offset.

CONCLUSION

Contrary to the traditional recommendation, the required WBGT offset was &gt;3°C in temperate conditions (&lt;27°C WBGT), particularly for moderate and heavy work. In contrast, a lower WBGT offset could be applied during light work and moderate work in low levels of coverage.

APPLICATION

Correct WBGT offsets are important for enabling adequate risk management strategies for mitigating risks of exertional heat illness.

How Do Joint Kinematics and Kinetics Change When Walking Overground with Added Mass on the Lower Body?

Lower-limb exoskeletons, regardless of their control strategies, have been shown to alter a user's gait just by the exoskeleton's own mass and inertia. The characterization of these differences in joint kinematics and kinetics under exoskeleton-like added mass is important for the design of such devices and their control strategies. In this study, 19 young, healthy participants walked overground at self-selected speeds with six added mass conditions and one zero-added-mass condition. The added mass conditions included +2/+4 lb on each shank or thigh or +8/+16 lb on the pelvis. OpenSim-derived lower-limb sagittal-plane kinematics and kinetics were evaluated statistically with both peak analysis and statistical parametric mapping (SPM). The results showed that adding smaller masses (+2/+8 lb) altered some kinematic and kinetic peaks but did not result in many changes across the regions of the gait cycle identified by SPM. In contrast, adding larger masses (+4/+16 lb) showed significant changes within both the peak and SPM analyses. In general, adding larger masses led to kinematic differences at the ankle and knee during early swing, and at the hip throughout the gait cycle, as well as kinetic differences at the ankle during stance. Future exoskeleton designs may implement these characterizations to inform exoskeleton hardware structure and cooperative control strategies.

Biomechanical knee energy harvester: Design optimization and testing

Biomechanical energy harvesters are designed to generate electrical energy from human locomotion (e.g., walking) with minimal or no additional effort by the users. These harvesters aim to carry out the work of the muscles during phases in locomotion where the muscles are acting as brakes. Currently, many harvesters focus on the knee joint during late swing, which is only one of three phases available during the gait cycle. For the device to be successful, there is a need to consider design components such as the motor/generator and the gear ratio. These components influence the amount of electrical energy that could be harvested, metabolic power during harvesting, and more. These various components make it challenging to achieve the optimal design. This paper presents a design of a knee harvester with a direct drive that enables harvesting both in flexion and extension using optimization. Subsequently, two knee devices were built and tested using five different harvesting levels. Results show that the 30% level was the best, harvesting approximately 5 W of electricity and redacting 8 W of metabolic energy compared to walking with the device as a dead weight. Evaluation of the models used in the optimization showed a good match to the system model but less for the metabolic power model. These results could pave the way for an energy harvester that could utilize more of the negative joint power during the gait cycle while reducing metabolic effort.

A Passive Back-Support Exoskeleton for Manual Materials Handling: Reduction of Low Back Loading and Metabolic Effort during Repetitive Lifting

OCCUPATIONAL APPLICATIONSGlobalization and eCommerce continue to fuel unprecedented growth in the logistics and warehousing markets. Simultaneously, the biggest bottleneck for these industries is their human capital. Where automation and robotic solutions fail to deliver a return on investment, humans frequently take over handling tasks that place harmful loads and strains on the body. Occupational exoskeletons can reduce fatigue and strain by supporting the lower spine and are designed to prevent work-related musculoskeletal disorders and other injuries. They are a mid- to long-term investment for industries to improve ergonomic conditions in workplaces, with the potential for reducing absences from work, sick days logged, and workers compensation claims. To examine the effectiveness of the newly introduced Paexo Back exoskeleton, a study was completed with 10 participants who completed manual load handling tasks with and without the exoskeleton. Key findings include significant reductions in metabolic effort and low back loading when the exoskeleton is worn.

Modeling the Metabolic Costs of Heavy Military Backpacking

Introduction

Existing predictive equations underestimate the metabolic costs of heavy military load carriage. Metabolic costs are specific to each type of military equipment, and backpack loads often impose the most sustained burden on the dismounted warfighter.

Purpose

This study aimed to develop and validate an equation for estimating metabolic rates during heavy backpacking for the US Army Load Carriage Decision Aid (LCDA), an integrated software mission planning tool.

Methods

Thirty healthy, active military-age adults (3 women, 27 men; age, 25 ± 7 yr; height, 1.74 ± 0.07 m; body mass, 77 ± 15 kg) walked for 6–21 min while carrying backpacks loaded up to 66% body mass at speeds between 0.45 and 1.97 m·s⁻¹. A new predictive model, the LCDA backpacking equation, was developed on metabolic rate data calculated from indirect calorimetry. Model estimation performance was evaluated internally by k-fold cross-validation and externally against seven historical reference data sets. We tested if the 90% confidence interval of the mean paired difference was within equivalence limits equal to 10% of the measured metabolic rate. Estimation accuracy and level of agreement were also evaluated by the bias and concordance correlation coefficient (CCC), respectively.

Results

Estimates from the LCDA backpacking equation were statistically equivalent (P < 0.01) to metabolic rates measured in the current study (bias, −0.01 ± 0.62 W·kg⁻¹; CCC, 0.965) and from the seven independent data sets (bias, −0.08 ± 0.59 W·kg⁻¹; CCC, 0.926).

Conclusions

The newly derived LCDA backpacking equation provides close estimates of steady-state metabolic energy expenditure during heavy load carriage. These advances enable further optimization of thermal-work strain monitoring, sports nutrition, and hydration strategies.

Stiffening the human foot with a biomimetic exotendon

Shoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human. Recent footwear design developments have shown that this approach may also produce metabolic energy savings. Here we test an alternative approach to modify the foot-ground interface by adding additional stiffness in parallel to the plantar aponeurosis, targeting the windlass mechanism. Stiffening the windlass mechanism by about 9% led to decreases in peak activation of the ankle plantarflexors soleus (~ 5%, p < 0.001) and medial gastrocnemius (~ 4%, p < 0.001), as well as a ~ 6% decrease in positive ankle work (p < 0.001) during fixed-frequency bilateral hopping (2.33 Hz). These results suggest that stiffening the foot may reduce cost in dynamic tasks primarily by reducing the effort required to plantarflex the ankle, since peak activation of the intrinsic foot muscle abductor hallucis was unchanged (p = 0.31). Because the novel exotendon design does not operate via the compression or bending of a bulky midsole, the device is light (55 g) and its profile is low enough that it can be worn within an existing shoe.

Optimized hip-knee-ankle exoskeleton assistance reduces the metabolic cost of walking with worn loads

BACKGROUND

Load carriage is common in a wide range of professions, but prolonged load carriage is associated with increased fatigue and overuse injuries. Exoskeletons could improve the quality of life of these professionals by reducing metabolic cost to combat fatigue and reducing muscle activity to prevent injuries. Current exoskeletons have reduced the metabolic cost of loaded walking by up to 22% relative to walking in the device with no assistance when assisting one or two joints. Greater metabolic reductions may be possible with optimized assistance of the entire leg.

METHODS

We used human-in the-loop optimization to optimize hip-knee-ankle exoskeleton assistance with no additional load, a light load (15% of body weight), and a heavy load (30% of body weight) for three participants. All loads were applied through a weight vest with an attached waist belt. We measured metabolic cost, exoskeleton assistance, kinematics, and muscle activity. We performed Friedman's tests to analyze trends across worn loads and paired t-tests to determine whether changes from the unassisted conditions to the assisted conditions were significant.

RESULTS

Exoskeleton assistance reduced the metabolic cost of walking relative to walking in the device without assistance for all tested conditions. Exoskeleton assistance reduced the metabolic cost of walking by 48% with no load (p = 0.05), 41% with the light load (p = 0.01), and 43% with the heavy load (p = 0.04). The smaller metabolic reduction with the light load may be due to insufficient participant training or lack of optimizer convergence. The total applied positive power was similar for all tested conditions, and the positive knee power decreased slightly as load increased. Optimized torque timing parameters were consistent across participants and load conditions while optimized magnitude parameters varied.

CONCLUSIONS

Whole-leg exoskeleton assistance can reduce the metabolic cost of walking while carrying a range of loads. The consistent optimized timing parameters across participants and conditions suggest that metabolic cost reductions are sensitive to torque timing. The variable torque magnitude parameters could imply that torque magnitude should be customized to the individual, or that there is a range of useful torque magnitudes. Future work should test whether applying the load to the exoskeleton rather than the person's torso results in larger benefits.

Transtibial limb loss does not increase metabolic cost in three-dimensional computer simulations of human walking

Loss of a lower limb below the knee, i.e., transtibial limb loss, and subsequently walking with a prosthesis, is generally thought to increase the metabolic cost of walking vs. able-bodied controls. However, high-functioning individuals with limb loss such as military service members often walk with the same metabolic cost as controls. Here we used a 3-D computer model and optimal control simulation approach to test the hypothesis that transtibial limb loss in and of itself causes an increase in metabolic cost of walking. We first generated N = 36 simulations of walking at 1.45 m/s using a “pre-limb loss” model, with two intact biological legs, that minimized deviations from able-bodied experimental walking mechanics with minimum muscular effort. We then repeated these simulations using a “post-limb loss” model, with the right leg’s ankle muscles and joints replaced with a simple model of a passive transtibial prosthesis. No other changes were made to the post-limb loss model’s remaining muscles or musculoskeletal parameters compared to the pre-limb loss case. Post-limb loss, the gait deviations on average increased by only 0.17 standard deviations from the experimental means, and metabolic cost did not increase (3.58 ± 0.10 J/m/kg pre-limb loss vs. 3.59 ± 0.12 J/m/kg post-limb loss, p = 0.65). The results suggest that transtibial limb loss does not directly lead to an increase in metabolic cost, even when deviations from able-bodied gait mechanics are minimized. High metabolic costs observed in individuals with transtibial limb loss may be due to secondary changes in strength or general fitness after limb loss, modifiable prosthesis issues, or to prioritization of factors that affect locomotor control other than gait deviations and muscular effort.

Effects of modern military backpack loads on walking speed and cardiometabolic responses of US Army Soldiers

INTRODUCTION

Military leaders must understand how modern military equipment loads affect trade-offs between movement speed and physiological strain to optimize pacing strategies.

PURPOSE

To evaluate the effects of load carried in a recently developed military backpack on the walking speed and cardiometabolic responses of dismounted warfighters.

METHODS

Fifteen soldiers (1 woman, 14 men; age, 22 ± 2 years; height, 173 ± 7 cm; body mass (BM), 73 ± 10 kg) completed incremental walking tests with four external load conditions (0, 22, 44, or 66% BM) using the US Army's newest backpack: the Modular Lightweight Load-Carrying Equipment 4000 (MOLLE 4000). Oxygen uptake (V̇O₂) and heart rate (HR) were evaluated relative to maximal values (V̇O_2max and HR_max respectively). Testing ceased when participants completed the highest tested speed (1.97 m s^-1), exceeded a respiratory exchange ratio (RER) of 1.00, or reached volitional exhaustion.

RESULTS

Peak speed significantly decreased (p < 0.03) with successively heavier loads (0% BM, 1.95 ± 0.06 m s^-1; 22% BM, 1.87 ± 0.10 m s^-1; 44% BM, 1.69 ± 0.13 m s^-1; 66% BM, 1.48 ± 0.13 m s^-1). Peak V̇O₂ was significantly lower (p < 0.01) with 0% BM (47 ± 5% V̇O_2max) than each load (22% BM, 58 ± 8% V̇O_2max; 44% BM, 63 ± 10% V̇O_2max; 66% BM, 61 ± 11% V̇O_2max). Peak HR was significantly lower (p < 0.01) with 0% BM (71 ± 5% HR_max) versus each load (22% BM, 83 ± 6% HR_max; 44% BM, 87 ± 6% HR_max; 66% BM, 88 ± 6% HR_max).

CONCLUSION

Overburdened warfighters suffer severe impairments in walking speed even when carrying recently developed military load carriage equipment. Our results suggest that the relative work intensity of heavy load carriage may be better described when expressed relative to HR_max versus V̇O_2max.

Development and Investigation of a Wearable Aid for a Load Carriage Task

Anterior load carriage tasks are common and can lead to musculoskeletal disorders such as lower back pain. The objectives of this study were to develop a wearable carriage aid and examine its effectiveness on physical demands while considering the potential moderating influence of the carried load. The study consisted of two within-subject factors: device and load. For the former, two levels were tested: with and without the device worn. For the latter, two loads were examined: 15 and 30% of each individual’s body mass. Sixteen participants walked on a treadmill for five minutes at a constant speed for each condition. Physical demands were quantified using objective (EMG-based) and subjective (discomfort) measures. Wearing the device reduced static and median anterior deltoid, trapezius, and biceps brachii muscle activations. Increasing the carried load increased most physical demand measures. Two significant Device×Load interactions were observed; for the anterior deltoid and trapezius median activation measures, the influence of increasing load was lower when the device was worn. While slightly increasing perceived discomfort in the lower back, wearing the device reduced shoulder, neck, and hand/wrist discomfort. While the study demonstrated a potential for the device, future work is required under more realistic and diverse testing conditions.

A Variable Stiffness Actuator Module With Favorable Mass Distribution for a Bio-inspired Biped Robot

Achieving human-like locomotion with humanoid platforms often requires the use of variable stiffness actuators (VSAs) in multi-degree-of-freedom robotic joints. VSAs possess 2 motors for the control of both stiffness and equilibrium position. Hence, they add mass and mechanical complexity to the design of humanoids. Mass distribution of the legs is an important design parameter, because it can have detrimental effects on the cost of transport. This work presents a novel VSA module, designed to be implemented in a bio-inspired humanoid robot, Binocchio, that houses all components on the same side of the actuated joint. This feature allowed to place the actuator's mass to more proximal locations with respect to the actuated joint instead of concentrating it at the joint level, creating a more favorable mass distribution in the humanoid. Besides, it also facilitated it's usage in joints with centralized multi-degree of freedom (DoF) joints instead of cascading single DoF modules. The design of the VSA module is presented, including it's integration in the multi-DoFs joints of Binocchio. Experiments validated the static characteristics of the VSA module to accurately estimate the output torque and stiffness. The dynamic responses of the driving and stiffening mechanisms are shown. Finally, experiments show the ability of the actuation system to replicate the envisioned human-like kinematic, torque and stiffness profiles for Binocchio.

Metabolic Costs of Standing and Walking in Healthy Military-Age Adults: A Meta-regression

INTRODUCTION

The Load Carriage Decision Aid (LCDA) is a U.S. Army planning tool that predicts physiological responses of soldiers during different dismounted troop scenarios. We aimed to develop an equation that calculates standing and walking metabolic rates in healthy military-age adults for the LCDA using a meta-regression.

METHODS

We searched for studies that measured the energetic cost of standing and treadmill walking in healthy men and women via indirect calorimetry. We used mixed effects meta-regression to determine an optimal equation to calculate standing and walking metabolic rates as a function of walking speed (S, m·s). The optimal equation was used to determine the economical speed at which the metabolic cost per distance walked is minimized. The estimation precision of the new LCDA walking equation was compared with that of seven reference predictive equations.

RESULTS

The meta-regression included 48 studies. The optimal equation for calculating normal standing and walking metabolic rates (W·kg) was 1.44 + 1.94S + 0.24S. The economical speed for level walking was 1.39 m·s (~ 3.1 mph). The LCDA walking equation was more precise across all walking speeds (bias ± SD, 0.01 ± 0.33 W·kg) than the reference predictive equations.

CONCLUSION

Practitioners can use the new LCDA walking equation to calculate energy expenditure during standing and walking at speeds <2 m·s in healthy, military-age adults. The LCDA walking equation avoids the errors estimated by other equations at lower and higher walking speeds.

Preferred walking speed on rough terrain; is it all about energetics?

Humans have evolved the ability to walk very efficiently. Further, humans prefer to walk at speeds that approximately minimize their metabolic energy expenditure per unit distance (i.e. gross cost of transport, COT). This has been found in a variety of population groups and other species. However, these studies were mostly performed on smooth, level ground or on treadmills. We hypothesized that the objective function for walking is more complex than only minimizing the COT. To test this idea, we compared the preferred speeds and the relationships between COT and speed for people walking on both a smooth, level floor and a rough, natural terrain trail. Rough terrain presumably introduces other factors, such as stability, to the objective function. 10 healthy men walked on both a straight, flat, smooth floor and on an outdoor trail strewn with rocks and boulders. In both locations, subjects performed 5-7 trials at different speeds relative to their preferred speed. The COT-speed relationships were similarly U-shaped for both surfaces, but the COT values on rough terrain were approximately 115% greater. On the smooth surface, the preferred speed (1.24+/−0.17 m/sec) was found to be statistically not different (p-value =0.09) than the speed that minimized COT (1.34 +/− 0.03 m/sec). On rough terrain, the preferred speed (1.07+/−0.05 m/sec) was slower than the COT minimum speed (1.13 +/− 0.07 m/sec) and was statistical significant (p-value=0.02). Since near the optimum speed the COT function is very shallow, these changes in speed result in small change in COT (0.5%). It appears that the objective function for speed preference when walking on rough terrain includes COT and additional factors such as stability.

Lower-limb joint work and power are modulated during load carriage based on load configuration and walking speed

Soldiers regularly transport loads weighing >20 kg at slow speeds for long durations. These tasks elicit high energetic costs through increased positive work generated by knee and ankle muscles, which may increase risk of muscular fatigue and decrease combat readiness. This study aimed to determine how modifying where load is borne changes lower-limb joint mechanical work production, and if load magnitude and/or walking speed also affect work production. Twenty Australian soldiers participated, donning a total of 12 body armor variations: six different body armor systems (one standard-issue, two commercially available [cARM1-2], and three prototypes [pARM1-3]), each worn with two different load magnitudes (15 and 30 kg). For each armor variation, participants completed treadmill walking at two speeds (1.51 and 1.83 m/s). Three-dimensional motion capture and force plate data were acquired and used to estimate joint angles and moments from inverse kinematics and dynamics, respectively. Subsequently, hip, knee, and ankle joint work and power were computed and compared between armor types and walking speeds. Positive joint work over the stance phase significantly increased with walking speed and carried load, accompanied by 2.3-2.6% shifts in total positive work production from the ankle to the hip (p < 0.05). Compared to using cARM1 with 15 kg carried load, carrying 30 kg resulted in significantly greater hip contribution to total lower-limb positive work, while knee and ankle work decreased. Substantial increases in hip joint contributions to total lower-limb positive work that occur with increases in walking speed and load magnitude highlight the importance of hip musculature to load carriage walking.

Does Wearing a Portable Metabolic Unit Affect Youth's Physical Activity or Enjoyment During Physically Active Games or Video Games?

Portable metabolic units (PMUs) are used to assess energy expenditure, with the assumption that physical activity level and enjoyment are unaffected due to the light weight and small size.

PURPOSE

To assess differences in physical activity level and enjoyment while wearing and not wearing a PMU.

METHOD

Youth (8-17 y; N = 73) played children's games or active video games while wearing and not wearing a PMU (crossover design). Participants wore an accelerometer and heart rate monitor and responded to questions about enjoyment on a facial affective scale. A repeated-measures analysis of variance was used to determine if accelerometer measures, heart rate, or enjoyment differed between conditions overall and by sex and weight status.

RESULTS

Steps per minute were lower while wearing the PMU than not wearing the PMU (40 vs 44, P = .03). There was an interaction between PMU condition and weight status for enjoyment (P = .01), with overweight participants reporting less enjoyment when wearing the PMU compared with not wearing the PMU (72 vs 75 out of 100). Heart rate, vector magnitude, and counts per minute were not different.

CONCLUSION

There may be psychosocial effects of wearing the PMU, specifically in overweight participants. Activity level was minimally affected, but the practical significance for research is still unknown.

The influence of increased distal loading on metabolic cost, efficiency, and kinematics of roller ski skating

The purpose of the present study was to examine the influence of increased loading of the roller ski on metabolic cost, gross efficiency, and kinematics of roller ski skating in steep and moderate terrain, while employing two incline-specific techniques. Ten nationally ranked male cross-country skiers were subjected to four 7-minute submaximal intervals, with 0, 0.5, 1.0, and 1.5 kg added beneath the roller-ski in a randomized order. This was done on two separate days, with the G2 skating at 12% incline and 7 km/h speed and G3 skating at 5% incline and 14 km/h speed, respectively. At 12% incline, there was a significant increase in metabolic rate and a decrease in gross efficiency with added weight (P<0.001 and P = 0.002). At 5% incline, no change in metabolic rate or gross efficiency was found (P = 0.89 and P = 0.11). Rating of perceived exertion (RPE) increased gradually with added weight at both inclines (P>0.05). No changes in cycle characteristics were observed between the different ski loadings at either incline, although the lateral and vertical displacements of the foot/skis were slightly altered at 12% incline with added weight. In conclusion, the present study demonstrates that increased loading of the ski increases the metabolic cost and reduces gross efficiency during steep uphill roller skiing in G2 skating, whereas no significant effect was revealed when skating on relatively flat terrain in G3. Cycle characteristics remained unchanged across conditions at both inclines, whereas small adjustments in the displacement of the foot coincided with the efficiency changes in uphill terrain. The increased RPE values with added ski-weight at both inclines indicates that other factors than those measured here could have influenced effort and/or fatigue when lifting a heavier ski.

Factors determining workers' pace while conducting continuous sequential lifting, carrying, and lowering tasks

To plan a new manual material handling work process, it is necessary to predict the times required to complete each task. Current time prediction models lack validity when the handled object's mass exceeds 2 kg. In this study, we investigated the effect of workplace design parameters on continuous sequential lifting, carrying, and lowering of boxes weighing from 2 kg to 14 kg. Both laboratory and field experiments were conducted. Results revealed that the box's weight and the lifting and lowering heights influenced the tasks' times. Further, the time to perform a task was influenced by the performance of other tasks in the same work process. New time prediction models were developed using the laboratory experiment data. Our models were found to be more accurate on average than the Maynard Operation Sequence Technique (MOST) and Methods Time Measurement (MTM-1) by 42% and 20%, respectively, for predicting the times of real workers at an actual workplace.

The Pandolf load carriage equation is a poor predictor of metabolic rate while wearing explosive ordnance disposal protective clothing

This investigation aimed to quantify metabolic rate when wearing an explosive ordnance disposal (EOD) ensemble (~33kg) during standing and locomotion; and determine whether the Pandolf load carriage equation accurately predicts metabolic rate when wearing an EOD ensemble during standing and locomotion. Ten males completed 8 trials with metabolic rate measured through indirect calorimetry. Walking in EOD at 2.5, 4.0 and 5.5km·h^-1 was significantly (p < 0.05) greater than matched trials without the EOD ensemble by 49% (127W), 65% (213W) and 78% (345W), respectively. Mean bias (95% limits of agreement) between predicted and measured metabolism during standing, 2.5, 4 and 5.5km·h^-1 were 47W (19 to 75W); -111W (-172 to -49W); -122W (-189 to -54W) and -158W (-245 to -72W), respectively. The Pandolf equation significantly underestimated measured metabolic rate during locomotion. These findings have practical implications for EOD technicians during training and operation and should be considered when developing maximum workload duration models and work-rest schedules. Practitioner Summary: Using a rigorous methodological design we quantified metabolic rate of wearing EOD clothing during locomotion. For the first time we demonstrated that metabolic rate when wearing this ensemble is greater than that predicted by the Pandolf equation. These original findings have significant implications for EOD training and operation.

Effect of an on-hip load-carrying belt on physiological and perceptual responses during bimanual anterior load carriage

Manual load carriage continues to be a major contributor of musculoskeletal injury. This study investigates the physiological and subjective effects of an on-hip load-carrying belt (HLCB) during bimanual anterior load carriage. Fifteen healthy male participants walked on a level ground treadmill at 4.5 km/h for 5 min carrying 5, 10 and 15 kg loads with hands and arms in front of the body, with and without using the HLCB (WD and ND). Heart rate, normalized oxygen uptake, minute ventilation and, central and peripheral ratings of perceived exertion were the dependent variables. The mean heart rate, normalized oxygen uptake, minute ventilation and peripheral rating of perceived exertion increased significantly with load under both WD and ND conditions. At a load of 15 kg, the mean heart rate, normalized oxygen uptake, minute ventilation and peripheral rating of perceived exertion were significantly lower by 6.6%, 8.0%, 11.8% and 13.9% respectively in WD condition when compared to the ND condition. There was no significant difference between WD and ND conditions with 5 or 10 kg load. It can be concluded that the HLCB could reduce a person's physiological and peripheral perceptual responses when walking on a level ground treadmill at 4.5 km/h with a load of 15 kg. Using a HLCB or similar device is therefore recommended for bimanual anterior load carriage for loads of 15 kg or probably larger.

Harvesting biomechanical energy or carrying batteries? An evaluation method based on a comparison of metabolic power

Background

Harvesting energy from human motion is an innovative alternative to using batteries as a source of electrical power for portable devices. Yet there are no guidelines as to whether energy harvesting should be preferred over batteries. This paper introduces an approach to determine which source of energy should be preferred. The proposed approach compares the metabolic power while harvesting energy and while using batteries (or any other power supply, e.g., solar panels), which provide equal amount of energy. Energy harvesting is preferred over batteries if the metabolic power required to harvest the energy is lower than that required to carry the batteries. Metabolic power can be experimentally measured. However, for design purposes, it is essential to assess differences in metabolic power as a function of the device parameters.

The model

To this end, based on the proposed approach, we develop a mathematical model that considers the following parameters: the device’s mass, its location on the human body, the electrical power output, cost of harvesting (COH), walking time, and the specific energy of the battery.

Method

We apply the model in two ways. First, we conduct case studies to examine current ankle, knee, and back energy harvesting devices, and assess the walking times that would make these devices preferable over batteries. Second, we conduct a design scenarios analysis, which examines future device developments.

Results

The case studies reveal that to be preferred over batteries, current harvesting devices located on the ankle, knee, or back would require walking for 227 hours, 98 hours, or 260 hours, respectively. This would replace batteries weighing 6.81 kg (ankle), 5.88 kg (knee), or 2.6 kg (back). The design scenarios analysis suggests that for harvesting devices to be beneficial with less than 25 walking hours, future development should focus on light harvesting devices (less than 0.2 kg) with low COH (equal or lower than 0). Finally, a comparison with portable commercial solar panels reveals that under ideal sun exposure conditions, solar panels outperform the current harvesting devices.

Conclusions

Our model offers a tool for assessing the performance of energy harvesting devices.

Electronic supplementary material

The online version of this article (doi:10.1186/s12984-015-0023-7) contains supplementary material, which is available to authorized users.